a) Field of the Invention
The present invention concerns a process of cutting out, with mechanical means, polymer electrolyte multi-layer electrochemical generators, in which the anode is preferably based on metallic lithium or compounds or alloys thereof. The invention is also concerned with certain designs of solid state multilayer layer batteries, produced according to the cutting process of the invention.
b) Description of the Prior Art
The manufacture of polymer electrolyte electrochemical generators or batteries in the form of thin films requires the production of multi-layer battery designs and involves the development of a surface which can reach the power and energy capacities required by the users. Generally, these generators are obtained by rolling or continuously or step-wise stacking important surfaces of juxtaposed films of anode, electrolyte, cathode, collector and, if necessary, electrically insulating materials. Reference will, for example, be made to U.S. Pat. No. 5,100,746 of Mar. 31, 1992.
On the other hand, it is known that thin laminated polymer electrolyte lithium batteries, which have been cut out with mechanical means, have the property of self-healing by local dissolution of lithium. However, when polymer electrolyte lithium batteries are in the form of multi-layer assemblies of thin laminated batteries, obtained either by rolling thin laminated batteries or by stacking thin laminated batteries in a parallel or bipolar arrangement, in which at least one rigid metallic collector is present in the thin laminated battery, it is difficult to achieve a mechanical cutting of such an assembly if the latter comprises more than one layer of thin laminated battery. Indeed, in the presence of a plurality of metallic layers such as lithium films and current collectors in the case of the films of the cathode, mechanical cutting produces local stresses and mechanical deformations due to local pressure which causes permanent short circuits in the battery.
It has been shown in U.S. Pat. No. 5,250,784 that it is possible to manufacture small individual batteries by cutting with laser. Moreover, in Canadian Patent Application Nos. 2,203,490 and 2,203,869 filed by the Applicant on Apr. 23 and 28, 1997 respectively, it has been shown that it is even possible to cut a single layer of laminated battery by a simple mechanical cutting operation. Cutting with laser, which vaporizes the materials without producing mechanical deformations, may itself generate residues which are sometimes conductive, for example, carbonized deposits which could cause short circuits. The technique of cutting with laser is therefore difficult to apply for parallel or bipolar multi-layer batteries. With respect to the mechanical cutting of one single layer, it is hard to apply to a multi-layer battery by reason of the cumulative mechanical deformations locally generated during the cutting operation, naturally resulting in the production of short circuits or weak points causing short circuits during consecutive cycles of charge/discharge.
An advantageous way of producing a multi-layer generator often consists in rolling or stacking the elements in parallel while allowing the respective collectors of the anode and the cathode to protrude on different lateral edges, so as to permit collection of the entire surface of the electrodes with lateral connections. Reference is made to U.S. Pat. No. 5,415,954 of May 16, 1995.
On the other hand, a battery design which is of particular interest, because it combines ease of manufacturing and volumic compactness is the one obtained by flat rolling, which is well known to those skilled in the art. The disadvantages associated with this design are, for example, the presence of curvature areas where, locally, the surfaces and current densities are uneven and where the pressure resulting from volume variations of the assembly cannot be compensated, which inevitably produces local stresses during consecutive cycles of charge/discharge. In a very general manner, the presence of these non-homogeneous areas creates zones of weakness of the electrical field within the generator, which causes accelerated aging. An elegant manner of solving this type of problem would be to be able to cut these curvature areas after preparing the assembly so as to produce a multi-layer assembly which is totally homogeneous along its entire surface. However, as mentioned above, the processes which are available cannot produce the desired results without serious disadvantages.
It is therefore an object of the invention to solve the type of problems mentioned above in an elegant manner, while optimizing the designs and performances of electrochemical generators.
It is another object of the present invention to provide a process enabling to cut out curvature areas of a flat roll, after assembly thereof, so as to produce a multi-layer assembly which is totally homogeneous along its entire surface.
It is also an object of the invention to provide batteries of smaller dimensions by cutting a multi-layer battery of a larger dimension, wherein the operation is a sharp cutting which utilizes localized abrasive mechanical means, such as water jet cutting containing an abrasive agent, or oil cutting containing an abrasive agent, or cutting with a diamond-containing wire, and without exercising pressure on the metallic collectors nor producing sufficient mechanical deformations to generate short circuits or weak points.
Another object of the invention resides in the demonstration that mechanical cutting operations such as but not limited to shear cutting, crush cutting or punching are possible under certain conditions and that it is possible to produce generators which are more compact and more performing during cycling through adequate control of the mechanical strengthening of the multi-layer battery and of the cutting conditions.
Another object of the invention is to provide for a step of mechanical strengthening of the multi-layer assembly which makes the films unitary with one another and the assembly relatively impervious, and which permits the use of a reactive liquid such as water.
Another object of the invention resides in the provision of a process of cutting using a water jet, which technology, up to now, was considered to be totally incompatible with a device consisting of thin films additionally including a lithium anode.
Another object of the invention resides in the provision of multi-layer designs of generators which are particularly advantageous as compared to the prior art, with respect to their performances as well as their ease of manufacturing.
The invention concerns a process for preparing an all-solid lithium or sodium electrochemical generator, comprising the following steps:
(a) preparing a stack of battery laminate, each comprising anode, polymer electrolyte, cathode, and collector films, and optionally an insulating film, and
(b) cutting the assembly obtained in (a) in predetermined shapes by utilizing mechanical means,
wherein the stack of battery laminate is prepared under conditions adapted to constitute a rigid substantially impervious monoblock assembly, in which the films are all adherent to each other, and the mechanical cutting is carried out without macroscopic deformation of the films constituting the assembly and without inducing permanent short circuits or weak points.
In the present specification and in the appended claims, the term xe2x80x9cmacroscopic deformationxe2x80x9d means a deformation which is visible to the naked eye. In other words, if some minute deformation takes place which is not visible to the naked eye, according to the invention, this means that there is no macroscopic deformation of the film.
According to a preferred embodiment, the cutting operation may be carried out by means of a jet of a reactive or non-reactive fluid, and, if needed, the jet may include a solid abrasive material which is dispersed within the fluid. Examples of abrasive materials include, without limitation, silica sand, diamond powder, an abrasive of type Grit 80(trademark) of Barton Garnet, and others well known to those skilled in the art.
According to a preferred embodiment, the cutting operation is carried out by means of a movable abrasive wire which is capable of producing a cut without macroscopic deformation and short circuits. This abrasive wire may be a diamond-containing wire, and the wire itself is normally made of steel.
According to a preferred embodiment, the cutting operation may be carried out by mechanical means without macroscopic deformation within the multi-layer battery assembly. Examples of mechanical means include but are not limited to crush cutting, shear cutting and punching. Blades or dies used could be metallic or ceramic.
According to a preferred embodiment, the anode constituting each of the batteries is made of lithium or is a lithium-based compound or alloy, and should be capable of self-oxidation so as to constitute an electrically-insulating layer. Alternately, it should be capable of becoming chemically dissolved in the presence of the cathode following an accidental short circuit which may result from the cutting operation.
The stack of battery laminate, may be treated, according to the invention, to make the rigid monoblock assembly sufficiently impervious to prevent fluid penetration between layers of the monoblock assembly. This makes it possible according to the invention, during the cutting operation, to introduce a reactive fluid, such as water, alcohol, halogenated compounds, or sulfur or oxygen containing compounds, which are capable of oxidizing lithium and possibly of dissolving part of the lithium so as to prevent any electrical short circuits between the ends of the films of the anodes and of the conductive films of the cathode.
According to another embodiment, there is first prepared a multi-layer generator consisting of the succession of the following films: . . . anode, electrolyte, cathode, collector, cathode, electrolyte, anode . . . , and steps are taken so that these films all adhere to one another or are made adherent to one another, such as by an operation of hot pressing after assembly, or by utilizing a partially cross-linked electrolyte, or by utilizing cross-linkable or non-cross-linkable adhesion additives. In the same manner, a stack of batteries could also be prepared so as to obtain a multi-layer generator consisting of the succession of the following films: . . . anode, electrolyte, cathode, collector, insulating film, anode . . . A multi-layer generator could also be prepared consisting of the succession of the following films: . . . collector, anode, electrolyte, cathode, collector . . .
According to another embodiment, there is provided an elementary battery comprising films of anode, polymer electrolyte, cathode, collector and possibly a film of insulating material, the laminate of the elementary battery is rolled flat while laterally leaving an excess of anode and cathode collectors, and at least the areas of curvature are cut at the ends of the flat roll, according to step (b).
According to another embodiment, a longitudinal cut is made at the center of a symmetrical assembly so as to reduce losses due to the imprecision of the position of the films and the possible application of insulating bands to prevent possible lateral short circuits, such as described in U.S. Pat. No. 5,360,684 of Nov. 1, 1994 in the name of the Applicant.
According to another embodiment, part of the oxidized lithium is dissolved or mechanically removed so as to free the lateral ends of the sheets of the cathode.
According to another embodiment, a metal is pulverized on the face(s) where the collector is accessible, to provide for a lateral connection between the sheets of the collector of the cathode.
The invention also concerns an electrochemical generator comprising a monoblock multi-layer assembly in which the components comprise anode, polymer electrolyte, cathode, collector films and possibly insulating films, these films being laminated and unitary, adherent to one another and relatively impervious, the assembly including at least an end appearing as a uniform cut, the various films constituting the assembly having undergone no macroscopic deformation, the ends of the anode films possibly including an electrically insulating passivation film.